1. Field of the Invention
The invention relates to a transmission system comprising a transmitter for supplying a send signal to a transmission channel, the send signal being a combination of a main signal and an auxiliary signal, a receiver for deriving a received signal from the transmission channel corresponding to the send signal. The receiver includes a control system which comprises means for comparing the difference between a parameter of the received signal and a reference value, means for deriving a correction signal from the difference signal, and means for reducing the difference signal in response to the correction value, The receiver also comprises means for deriving the auxiliary signal from the difference signal, the frequency spectrum of the auxiliary signal being largely situated in the bandwidth of the control system.
The invention likewise relates to a receiver to be used in such a transmission system.
2. Description of the Related Art
A transmission system as defined in the opening paragraph is known from the published article "Channel Selection and Identification for Coherent Optical FDM Systems" by Q. Jiang in IEEE Photonits Technology Letters, Vol. 3, No 8, August 1991, pp. 267-268.
Such transmission systems may be used for transmitting signals through glass fibres, radio links or line links. Such systems may also be used for restoring signals that have been recorded on a recording medium such as, for example, magnetic tape or a magnetic or optical disc.
In systems of this type there is sometimes a desire to add an auxiliary signal, for example, in the form of a pilot tone. This is effected by varying a parameter of the send signal in response to the auxiliary signal. Such a pilot tone may be used, for example, for amplitude or frequency control of the received signal. Alternatively, it is conceivable that the pilot tone can be used for deriving a signal having a certain frequency and which is necessary for processing the main signal.
In order to avoid the reception of the main signal from being too strongly affected by the presence of the auxiliary signal (pilot tone), the receiver of the known transmission system comprises a control system to reduce the variations of the parameter of the received signal which is used for transmitting the auxiliary signal. In the known transmission system the auxiliary signal is transmitted by frequency modulation of an optical carrier. The control system in the receiver is in this case a frequency control system wherein the comparing means comprise a frequency discriminator and the correction means comprise the local laser the frequency of which is adjusted in response to the correction signal. This frequency adjustment of the local laser reduces frequency variation of the received signal due to the presence of the auxiliary signal to a value which no longer is a disturbing factor in the reception of the main signal. In order to make this possible the frequency spectrum of the auxiliary signal must lie in the bandwidth of the control system, because otherwise the control system will be incapable of keeping track of the variations caused by the auxiliary signal in the parameter of the received signal. In this connection the bandwidth of the control system is meant to be understood as the maximum frequency of variations of the characteristic magnitude of the received signal which variations can still be reduced by the control system.
In the transmission system known from said the above-identified article a pilot tone is added to identify the transmitter. Each transmitter in the transmission system sends out a pilot tone with a frequency associated with that particular transmitter. From the pilot tone frequency the receiver can therefore establish which transmitter is being received. However, it is impossible to transmit further information in addition to the transmitter identification.